Mixing circuit employing compensation for electron stream induction effect



Dec. 4, 1951 M. .1. o. STRUTT 2,577,489

MIXING CIRCUIT EMPLOYING COMPENSATION FOR ELECTRON STREAM INDUCTION EFFECT Filed Feb. 14, 1947 FIG.

FIG.3

M. .J .O STRUTT INVENTOR BY fi AGENT Patented Dec. 4, 1951 MIXING CIRCUIT EMPLOYING COMPENSA- TION FOR ELECTRON STREAM INDUCTION EFFECT Maximiliaan Julius Otto Strutt,

Eindhoven,

Netherlands, assignor to Hartford National Bank and Trust Company, Hartford, Conn., as

trustee Application February 14, 1947, Serial No. 728,707

In Belgium November 3, 1944 Section 1, Public Law 690, August 8, 1946 Patent expires November 3, 1964 6 Claims. (Cl. 250-20) This invention relates to a mixing circuit in which each of the signals to be mixed is supplied to a control grid of a multi-grid valve, one or more electrodes having a, positive bias being arranged between the two control grids, which are interconnected by way of a predominantly capacitative impedance, which is so proportioned as to compensate the signal transmission from the inner to the outer control grid which is due to the induction effect.

In mixing circuits of the above-indicated type a space charge, usually referred to as the virtual cathode, occurs in the neighbourhood of the outer control grid. The density of this space charge is governed at any time by the strength of the flow emitted from the cathode and consequently fluctuates in synchronism with the signal supplied to the inner control grid. This fluctuatin; space charge induces a corresponding fluctuating charge in the outer control grid, with the result that a current having the frequency of the signal supplied to the inner control grid occurs in the circuit of the outer control grid. This phenomenon is referred to as induction-effect.

The density of the said space charge is at least approximately in phase with the potential of the inner control grid. If consequently the inner control grid becomes more positive, the charge of the outer control grid also becomes more positive. If a capacity were connected between the two control grids the charge of the outer control grid would become more negative at an increase in a positive sense of the potential of the inner control grid. The induction effect is consequently equivalent to the presence of a negative capacity between the two control grids, it having however current passing through it only in one direction, to wit from the inner to the outer control grid. It is obvious that the said effect can be compensated for by the provision of a positive capacity, for example a condenser of about 2 ufds. between the two control grids.

In the frequency changing of ultra-high frequency oscillations the density of the said space discharge is delayed behind the volta e of the inner control grid due to the transit-time of the electrons. When compensating for the induction effect this delay should be taken into account. For this purpose it has been proposed to connect an ohmic resistance in parallel or in series with the said condenser. The impedance arranged between the two control grids for the sake of compensation has however, always a predominantly capacitative nature.

When using the compensation described, un-

due signal transmission from the inner to the outer control grid is no longer possible, but on the other hand there is the disadvantage that by way of the predominantly capacitative impedance undue signal transmission in the opposite sense, that is to say from the'outer to the inner control grid, is liable to occur.

The invention has for its object to obviate this disadvantage. According to the invention, for this purpose the circuit of an electrode'following the outer control grid and having a positive bias is so coupled with the circuit of the inner control grid as to compensate for the signal transmission from the outer to the inner control grid by way of the said predeminantly capacitative impedance.

Preferably, an impedance is arranged between the said electrode and the cathode, and a second impedance which is high compared with the firstmentioned impedance is connected between the said electrode and the inner control grid. In practice an ohmic resistance of a low multiple of 1000 ohms may be arranged between the said electrode and the cathode and a condenser having a capacity of about 1 ,uufd. is connected between the said electrode and the inner control grid.

When the inner control grid is followed by an oscillator anode the circuit of which is coupled with the circuit of the inner control grid for the purpose of generating the local oscillations, the said second impedance may also be arranged between the said electrode and the oscillator anode.

In order that the invention may be clearly understood and readily carried into effect it will now be described more fully with reference to the accompanying drawing.

Figure 1 represents a schematic diagram of the circuit according to the invention;

Fig. 2 represents a schematic diagram of a portion of the invention showing a further embodiment of the invention; and

Fig. 3 is a schematic diagram of a portion of the circuit according to the invention in accordance with a further modification.

Figure 1 of the drawing shows a mixing valve 1 of the octode type which comprises a cathode 2, an inner control grid 3, an oscillator anode 4, a screen grid 5, an outer control grid 6, a, second screen grid 1, a suppressor grid 8, which is connected to the cathode, and an anode 9. The circuit of the control grid 3 includes an oscillatory circuit I0 which is coupled with a feedback coil H included in the circuit of the oscillator anode 4 so that the local oscillations are set up in the circuit In. The signal oscillations are fed to the outer control grid by way of an input oscillatory circuit 12 and the intermediate frequency output voltage is obtained from an intermediate frequency circuit l3 included in anode circuit.

To compensate for the induction effect the control grids 3 and 6 have arranged between them an impedance of predominantly capacitative nature which is constituted by the series combination of condenser I4 and an ohmic resistance [5.

A disadvantage of this known circuit arrangement is that by way of the impedance l4, IS an undue signal transmission ensues from the outer control grid 6 to the inner control grid 3, which results inter alia in that frequency of the local oscillations varies with the bias of the outer control grid 6. According to the invention, this disadvantage is obviated by including in the circuit of the screen grid 1 a resistance I6 of a multiple of 1000 ohms which is not shunted so far as high-frequency currents are concerned and by inserting between the screen grid 1 and the control grid 3 a condenser 11 having a capacity of about 1 [L/Lfd. The resistance H3 has set up across it, inter alia, a voltage having the frequency of the received signal fed to the control grid 6 and being in phase opposition to the voltage of the control grid 6. Hence the condenser I! gives passage to a current passing to the control grid 3 and advancing by about 90 with respect to the voltage across the resistance i t and consequently lagging by about 90 with respect to the voltage of the control grid 6. Assuming proper proportioning of the elements l6 and I! the current that passes from the control grid 6 via the impedance [4, l5 to the control grid 3 can therefore be compensated by the current that passes through the condenser I! to the control grid 3 and this avoids undue reaction of the voltage supplied to the control grid 6 on the circuit of the control grid 3.

With the compensation described, the phase displacement between the current of the screen grid 7 and the voltage of the control grid 6 brought about by the transit-time of the electrons should, if necessary, be taken into account, as should also the phase displacement of the voltage transmitted by way of the impedance l4, l5 to the control grid 3, which phase displacement is brought about by the resistance 5. This may be eifected for example by connecting an ohmic resistance in series with the condenser I'l. v

Fig. 2 shows that instead of inserting the condenser l1 between the grids 3 and 1, it is also possible to arrange a predominantly inductive impedance [8 between the screen grid 7 and the oscillator anode 4. In this case, the compensation voltage is supplied to the control grid 3 by way of the feedback coil ll. As an alternative, this inductive impedance may be used jointly with the condenser l1 and this ensures the advantage that the compensation'can be rendered frequency-dependent according to a predetermined law in order to ensure particularly accurate compensation at certain frequencies, for example at the highest frequencies of the band of the signal oscillations.

As shown in Fig. 3 apart from a voltage having the frequency of the signal voltage supplied to the outer control grid 6 a voltage having the frequency of the local oscillations and an intermediate frequency voltage will be set up across the resistance 16. Under certain conditions this may give rise to undue back couplings. These the 4 difficulties can be avoided by substituting an oscillatory circuit I9, 20 of Fig. 3, tuned to the frequency of the received signal for the resistance IS.

The use of the invention has the advantage inter alia that the frequency of the oscillations set up in the circuit l0 varies only very little with the bias of the control grid Hi.

What I claim is:

1. An electrical circuit arrangement for mixing a first wave and a second wave to produce an intermediate frequency wave, comprising an electron discharge tube having in the order named a cathode, a first control grid electrode, a second control grid electrode, a screen grid electrode and an anode and having a further electrode, a capacitive element coupled between said control grid electrodes, means to apply said first wave to one of said control grid electrodes, oscillator means comprising the other of said control grid electrodes and said further electrode for generating said second wave, an impedance element coupled. to said screen grid electrode, a reactance element interconnecting the junction of said screen grid electrode and said impedance element to one of said electrodes of said oscillator means, and an output circuit coupled to said anode.

2. An electrical circuit arrangement for mixing a first wave and a second wave to produce an intermediate frequency wave, comprising an electron discharge tube having in the order named a cathode, a first control grid electrode, an oscillator anode, a second control grid electrode, a

' screen grid electrode and an anode, a capacitive element coupled between said control grid electrodes, means to apply said first wave to said second control grid electrode, oscillator means comprising said first control grid electrode and said oscillator anode for generating said second wave, an impedance element coupled to said screen grid electrode, a reactance element interconnecting the junction of said screen grid and said impedance element to one of said electrodes of said oscillator means, and an output circuit coupled to said anode.

3. An electrical circuit arrangement for mixing a first Wave and a second wave to produce an intermediate frequency wave, comprising an electron discharge tube having in the order named a cathode, a first control grid electrode, an oscillator anode, a second control grid electrode, a screen grid electrode and an anode, means to apply said first wave to said second control grid electrode, a capacitive element coupled between said control grid electrodes to apply to said first control grid electrode a first voltage having the frequency of said first wave, oscillator means comprising said first control grid electrode and said oscillator anode for generating said second wave, an impedance element coupled to said screen grid electrode, a reactance element interconnecting the junction of said screen grid and said impedance element to one of said electrodes of said oscillator means to apply to said one electrode a second voltage having the frequency of said first wave and in phase opposition to said first voltage, and an output circuit coupled to said anode.

4. An electrical circuit arrangement for mixing a first wave and a second wave to produce an intermediate frequency wave, comprising an electron discharge tube having in the order named a cathode, a first control grid electrode, an oscillator anode, a second control grid electrode,

a screen grid electrode and an anode, an impedance network comprising a first capacitive element and a first resistive element coupled between said control grid electrodes, means to apply said first Wave to said second control grid electrode, oscillator means comprising said first control grid electrode and said oscillator anode for generating said second wave, a second resistive element coupled to said screen grid electrode, a second capacitive element interconnecting the junction of said screen grid and said second resistive element to said first control grid electrode, and an output circuit coupled to said anode.

5. An electrical circuit arrangement for mixing a first wave and a second wave to produce an intermediate frequency wave, comprising an electron discharge tube having in the order named a cathode, a first control grid electrode, an oscillator anode, a second control grid electrode, a screen grid electrode and an anode, a capacitive element coupled between said control grid electrodes, means to apply said first wave to said second control grid electrode, oscillator means comprising said first control grid electrode and said oscillator anode for generating said second wave, a resistive element coupled to said screen grid electrode, an inductive element interconnecting the junction of said screen grid electrode and said resistive element to said oscillator anode, and an output circuit coupled to said anode.

6. An electrical circuit arrangement for mixing a first wave and a second wave to produce an intermediate frequency wave, comprising an electron discharge tube having in the order named a cathode, a first control grid electrode, an oscillator anode, a second control grid electrode, a screen grid electrode and an anode, a first capacitive element coupled between said control grid electrodes, means to apply said first wave to said second control grid electrode, oscillator means comprising said first control grid electrode and oscillator anode for generating said second Wave, an impedance network tuned to the frequency of said first wave coupled to said oscillator anode, a second capacitive element interconnecting the junction of said second screen grid electrode and said impedance network to said first control grid electrode, and an output circuit coupled to said anode.

MAXIMILIAAN JULIUS OTTO STRUTT.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PA'I'ENTS Number Name Date 2,033,986 Harris Mar. 17, 1936 2,050,474 Stone Aug. 11, 1936 2,12 ,283 Harris June 28, 1938 2,141,750 Herold Dec. 27, 1938 2,252,584 Strutt Aug. 12, 1941 2,314,785 Holland Mar. 23, 1943 FOREIGN PATENTS Number Country Date 437,804 Great Britain Oct. 31, 1935 

